Asbestos cement pipe being cured in an autoclave  - Rosato (C) InspectApedia Asbestos cement product curing & stabilization
asbestos cement roofing, siding & millboard production methods
     


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Curing & stabilization of cement-asbestos products: this article describes how asbestos cement roofing, siding, pipe, and other products were cured after the wet manufacturing process. Curing methods for asbestos-cement varied from autoclave, air-drying, to storage under-water!

Cement asbestos materials: this article series describes asbestos cement products & materials and the history of their production and use. This article includes text & data Adapted from Rosato (1959 out of print) on the production and use of asbestos-containing materials, adding photographs, inspection, repair and maintenance advice, and updates on asbestos hazards involved in the installation, repair, or demolition of asbestos cement products such as roofing and siding or asbestos millboard that remain in place on buildings.

Our page top photograph shows badly broken cement asbestos roofing shingles on a building in upstate New York.

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Curing Processes for Asbestos Cement Products

Cement asbestos roof shingles (C) Daniel Friedman

The following text is adapted & expanded from Rosato p. 73-75 [1] © 2013 InspectApedia.com

Three basic processes are available for curing the asbestos-cement products. These are similar to the procedures used for ordinary concrete materials, however, they are usually maintained under better actual control. The oldest procedure is wet curing.

The products are allowed to age in a warm humid atmosphere from 21 to 28 days.

[Click to enlarge any image]

Pipe products, under this system, are frequently stored under water after approximately 7 days of curing in air.

Typically, with cement products, the strength of asbestos-cement products increases as time passes. This condition is particularly true in the first year.

Occasionally, the products are cured for approximately a day in moist steam at atmospheric pressure. The average curing temperature is approximately 85 °C. The tempera-ture-time cycle varies and depends upon the product. Atmospheric steam curing is essentially an acceleration of normal curing; it cures products without requiring a long time cycle.

Asbestos cement pipe being cured in an autoclave  - Rosato (C) InspectApedia

Figure 3.3. Asbestos-cement product being cured in autoclave.

(Courtesy Johns-Manville Corp.)

Autoclave curing is more popular. See Figure 31. It uses saturated steam at pressures between 100 and 250 psi. This process is not simply an accelerated cure, giving an ade-quately cured product in one day, but it also alters the character of the material.

The Italian inventor, Dr. Morbelli, started the process of substituting finely ground, pure silica flour for a portion of the cement in the mixture.

The silica flour and the cement react under the curing conditions to form a crystalline product in place of the typical cement gel.

This method takes into consideration the chemical reac-tion between calcium hydroxide and silica which forms a hydrated calcium silicate. The silica is added to the com-position of the product to react with the calcium hydroxide which is formed when the cement reacts with water. Steam cure under pressure provides for the chemical reaction.

 

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